fix(device-pool): scope primary key by host and thread through capability_tags

- pooled_devices primary key changed from device_id alone to
  (host_id, device_id), so two hosts reporting the same local
  device_id no longer crash sync_host_devices() with an uncaught
  sqlite3.IntegrityError.
- Device gained a capability_tags field so DevicePool.sync_host_devices()
  can actually populate PooledDevice.capability_tags from a real host
  sync instead of always falling back to an empty list.

openspec: device-pool capability, archived change cloud-runtime

cloud/store.py

@@ -301,12 +301,13 @@ class CloudStore:
             connection.execute(
                 """
                 create table if not exists pooled_devices (
-                    device_id text primary key,
+                    device_id text not null,
                     host_id text not null,
                     driver_type text not null,
                     status text not null,
                     capability_tags_json text not null,
-                    synced_at text
+                    synced_at text,
+                    primary key (host_id, device_id)
                 )
                 """
             )